I purchased (may have got for free) the Windows 10 version many months ago and my daughters have loved it. To enable them to use the mods they see on YouTube videos, I understand I need to buy the Java edition. The added benefit would be I hope to set up a server so that they can play online with their cousin too.

I have a few questions:
Is there anything I can do to link my Windows 10 minecraft character (mainly the MC username and possibly the Microsoft account) to the new Java edition?

Is there any way to bring my (Windows 10) worlds into the Java Edition?

On an aside, can I use the free Windows 10 code (I get by purcahsing the Java version) on my dads Windows 10 computer (a different computer, different Microsoft account)?